It is particularly desirable at times to measure the cardiac output of a patient on a real time basis without employing invasive surgical techniques. Non-invasive measurement of cardiac output using Doppler ultrasound has been a goal for many years. Success has been reported using duplex imaging equipment by combining either echographic or M-mode measurement of aortic diameter as measured from the second or third intervostal space with Doppler velocity measurements performed from the suprasternal notch. Good correlation with thermodilution cardiac output measurements have been reported using these techniques. The primary drawback of these techniques is the requirement for relatively expensive equipment and highly skilled operators to perform the measurement.
Several previous approaches to making a dedicated instrument to compute cardiac output have used continuous wave Doppler from the suprasternal notch of a patient. These techniques relied upon CW Doppler to measure blood flow velocities in the ascending aorta, the aortic arch, or the descending aorta. Although these implementations proved quite successful in the hands of skilled operators, routine clinical application was made more difficult by the potential confusion of signals that may occur with CW Doppler. When measurements are attempted for flow in the ascending aorta, it is not uncommon to also find flow signals in the same area from the innominate artery on the right or the left carotid or subclavial arteries to the left. Although a window is generally available in which only the aortic signal can be found, an unskilled operator may have difficulty determining the difference between these signals from the aorta and other such signals available from the innominate carotid, or subclavian arteries. Flow signals measured from the aortic arch or descending aorta are potentially less representative of total cardiac output due to Doppler angle, as well as the lack of knowledge of the unknown percentage of flow which has been directed to the head. Flow measurements in the descending aorta may provide a good trend indicator but cannot readily provide absolute cardiac output information.
In the past, one method used to measure the cardiac output of a patient required a doctor to anesthetize a patient and insert an ultrasonic transducer probe in the esophagus near the aorta of the heart with much discomfort to the patient. Another method of cardiac output measurement involved the surgical insertion of a detector in the pulmonary artery of a patient. Use of this method was generally limited to extremely ill patients because it is a particularly risky operation.
Since continuous wave (CW) Doppler devices do not provide any range discrimination, no means is available for limiting the range distance from the transducer along the direction of the ultrasonic beam so that the measurement being taken may be optimized to correspond where the best reading for cardiac output velocity should be taken. This "best" location corresponds to a point approximately 2 cm above the annulus of the aortic valve at which point any turbulence associated with a normal aortic valve has diminished and a more or less uniform flow profile exists across the area of the aorta. It is not possible, when using continuous wave insonification, to uniquely determine the location along the ultrasonic beam from which a returning energy wave was reflected, to determine precisely where the reading came from, and if the reading represents blood flow in the ascending aorta. These systems require an ultrasound technician or a cardiologist to accurately analyze the output of the device. U.S. Pat. No. 4,509,526 discloses a device that uses continuous wave insonification to measure the blood flow velocity in the ascending aorta of a patient which is combined with a separate measurement of aortic diameter to provide a measure of cardiac output. This device requires a highly skilled ultrasound technician or a cardiologist to operate and to interpret the tracer display to receive a valid cardiac output velocity reading. The system does not have the ability to detect aortic velocity selectively at different distances from the transducer due to the fact that it uses continuous wave ultrasound. The device also does not have the capability to distinguish between signals representing noise or other reflected signals and
signals representing blood flow in the ascending aorta.